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The mojo API is a no-op other than on Android with |enabled| being
false. This patch refactors it and renames it into
|DisconnectSessionBridge| to make the behavior more explicit.

Bug: 814225
Change-Id: I786411375edd594806fb2f7bcdc7b884be4352fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1500016
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637847}